1. Field of the Invention
The present invention relates to a sandwich glass for use in a front glass or a side glass of an automobile, a window glass of a building, and the like.
2. Description of the Related Art
In general, polyvinylbutyral resin has hitherto been mainly used as an intermediate layer for a sandwich glass. However, polyvinylbutyral resin is thermoplastic and, therefore, has the following drawbacks:
(1) Since polyvinylbutyral resin has a relatively low softening point, sliding of glass plates or formation of bubbles in the sandwich glass occurs due to heat after the glass plates are stuck to the resin;
(2) Since polyvinylbutyral resin is apt to be influenced by moisture, when the sandwich glass is left to stand in a high humidity atmosphere over a long period of time, the resin gradually whitens from the peripheral portion of the sandwich glass, and also the adhesion force of the resin to the glass plate is reduced; and
(3) The impact fracture resistance of the sandwich glass depends upon temperature, and, in particular, the penetration resistance rapidly decreases in a temperature region exceeding room temperature, that is, at a temperature greater than or equal to approximately 30.degree. C.
In order to solve the above drawbacks of polyvinylbutyral resin, the inventors have previously proposed a sandwich glass obtained by interposing a thermosetting resin comprised of ethylene-vinyl acetate copolymer and an organic copolymer between glass plates and then conducting the thermosetting of the resin (Japanese Patent Application Laid-Open No. 57-196747).
Further, as a method of improving the impact fracture resistance and the penetration resistance of the sandwich glass, sandwich glasses have been proposed in which a thermosetting resin is mixed with a hydrocarbon resin of 200-50,000 weight average molecular weight. Patent applications for such sandwich glasses have already been filed (e.g., Japanese Patent Application Laid-Open No. 3-183645).
However, considering safety as a great requirement of the sandwich glass, the impact resistance, penetration resistance, and transparency over a long period of time cannot be said to be sufficient under wide conditions even in the inventors' previous inventions.
Further, with regard to improving the perspective distortion of the aforementioned sandwich glass, when the sandwich glass is used as the front glass in a regular automobile, in order for the sandwich glass not to present any problems with regard to the visibility of the driver, the value thereof obtained by a perspective distortion test must be less than or equal to 1.0 min of arc. When the reading portion of a bar code reader is used as the glass portion, mistakes in reading are prevented at values less than or equal to 0.8 min of arc. Sandwich glasses having such values are required.